Recently, as PC and Internet related industries grow more, a demand for larger scale data transfer has been rapidly increasing, and an accelerated optical data transfer is now prevailing. An optical waveguide serves as an optical Internet connection for optical data transfer. For instance, an optical apparatus such as an optical splitter (optical branching/coupling unit) has a tapered waveguide and a plurality of curved waveguides optically combined with each other in practical use. Such curved waveguides are typically constant in their respective core widths.
Some curved waveguide has its core width varied throughout the entire length, and one typical example is known as a curved waveguide that has its core width progressively enlarged and has both an outer and inner contours of its Y-branch portion shaped on the basis of a certain function in order to reduce an excess loss in the branching portion. See Japanese Patent No. 2589367.
In the case of an optical splitter made up of optical waveguides, especially in a part where two of curved waveguides are connected to a tapered waveguide, it is advantageous spacing those curved waveguides gradually farther away from each other from infinitely small interval in order to reduce an excess loss, but it is an unattainable job to locate the waveguides at such an infinitely small interval with a satisfactory yield because of a restraint of the manufacturing. Instead, disposing both the curved waveguides at a certain finitely small interval permits a reduction in adverse effects of an uneven configuration of the fabricated branching portion, and also permits a reduction in incomplete filling of a narrowed portion with cladding substance. The interval thus widened between the two waveguides unavoidably causes leak of light more into the cladding, and resultantly, causes an excess loss due to an excessive branching width.
The inventors of the present invention found a solution to the aforementioned prior art disadvantages by altering a curved shape of the optical waveguides and completed the invention. More preferably, the present invention overcomes the aforementioned disadvantages by altering a curved shape of an inner circumference of the optical waveguides. 